Overview data is most commonly found in raster formats. Overviews are resampled and lower-resolution versions of raster datasets that provide thumbnail views or simply faster-loading image views at different map scales. They are also known as pyramids, and the process of creating them is known as pyramiding an image. These overviews are usually preprocessed and stored with the full resolution data either embedded with the file or in a separate file.
The compromise of this convenience is that the additional images add to the overall file size of the dataset; however, they speed up image viewers. Vector data also has a concept of overviews, usually to give a dataset geographic context in an overview map. However, because vector data is scalable, reduced size overviews are usually created on the fly by software using a generalization operation, as mentioned in...